Daylight at the Bench: The Opening Fraction Needed for Fine Work, and a Test of the Claim That One Translucent Roof Sheet Daylights a Workshop

Translucent roof sheets are sold and fitted on the understanding that a single sheet turns a small workshop into a day-lit workspace. This paper tests that claim by computation for a 4.0 by 3.0 m workshop with a height of 2.7 m, a fine-work threshold of 500 lux at the bench, and the standard daylight factor method. One standard sheet of 2.0 by 0.9 m (1.8 m2, 15.0% of the floor area), with a stated transmittance of 0.60 for an aged sheet, gives an average daylight factor of 1.87% and a daylight factor at the bench of 1.31%. The bench illuminance is therefore 65.5 lux under a winter overcast sky of 5000 lux, 131.1 lux under a standard overcast sky of 10000 lux, and 262.1 lux under a bright overcast sky of 20000 lux — below even a 300 lux general-work level in every sky modelled, and below the 500 lux fine-work threshold by a factor of about four under the standard sky. Holding 500 lux at the bench requires a roof opening of 6.87 m2 (57.2% of the roof) under the standard sky, 3.43 m2 under the bright sky, and 13.73 m2 under the winter sky, which is more than the 12.0 m2 roof itself. In a stated hour-by-hour overcast model the single sheet provides 0 hours at or above 500 lux and 0 hours at or above 300 lux in winter, at the equinox, and in summer alike. The claim does not survive the arithmetic: one sheet provides useful background light, not a day-lit bench for fine work.